This invention concerns methods of and apparatus for the encoded transmissions of information in which message elements of equal length are interchanged in time at the transmitter by storage and delay and are re-interchanged at the receiver. In known methods of this kind the storage times are chosen so that no element of the original message (clear message) is omitted from the encoded message and also so that no two or more elements appear simultaneously.
In accordance with a known proposal in British Patent No. 1,353693 several stores are employed, the capacity of each corresponding to one message element. At equal time intervals, corresponding to the length of one message element, there is determined by a quasi-random control signal the number of that one of the stores from which an element of the encoded message is to be withdrawn and into which at the same time an element of the clear message is to introduced. In this method excessive storage times can be avoided by automatic withdrawal of a message element from any store when a predetermined maximum storage time has been reached. In this method of coding, the delay times of the message elements are not uniformly distributed: the probability decreases with increasing delay, while the extreme delay does not appear very frequently. It is true that a certain equalisation of this cryptologically undesirable distribution is possible by operating the individual storeswith unequal maximum delay times. The cost of carrying out this operation automatically is however considerable and in addition it is undesirable for reasons of cryptological security that the storage times shall be determined to an increased extent by the individual storage capacity and not in the first place by the quasi-random control signal.
According to other proposals, given British Patent No. 1,356,970, the message elements are stored in shifting registers, in which the input and withdrawal locations are determined by quasi-random control signals. After the withdrawal of an element the respective storage location thus remains temporarily empty. This leads to inefficient utilization of the available storage capacity and thus to relatively high cost.